An optical detection arrangement, such as is commonly used in automatic milking systems, typically includes a detector in the form of a diode detector or CCD array camera for detecting radiation that is emitted or reflected from an object of interest, and using the detected radiation to determine the distance between the object and the detector. Some systems may also have a radiation source, for example, a laser, used to illuminate objects of interest with suitable radiation. In most cases, the radiation detector and source, if present, are arranged in a housing behind a screen or window that protects the camera or diode from surrounding pollution. The window may be made of glass or other suitably transparent material through which visible light or other radiation can pass substantially without attenuation. In order for an optical detection system of this kind to operate reliably, it is important that the transmission of radiation through the window is not blocked by deposits such as dirt, dust or water. Deposits of any kind reduce the amount of optical radiation that can reach the detector and, in systems containing a source of radiation for illuminating an object, may also prevent an object from being illuminated adequately.
A particular challenge in a milking parlour is the amount of dirt and liquid that is present around the system. All equipment that comes into contact with the animals must be kept clean. Generally, this is achieved by subjecting the relevant elements of the milking system, such as teat cups, teat cleaning devices and even parts of the robot arm to a periodic wash with a cleaning or disinfecting liquid or at least with water. In addition, when an animal is to be milked, the teats must be washed and typically also disinfected after milking. During this process, droplets of liquid can become airborne and thus be likely to settle on the detector window. Milk may also leak from the teats of an animal both prior to and after milking. Since the optical detectors are often positioned close to the teats of an animal, milk can also land on the detector window. Additionally, the floor of the milking parlour is often wet as a result of the various cleaning operations, so that droplets can be thrown upwards by the movement of an animal's feet.
Existing systems have cleaning devices either provided separately from the detectors or connected to the detector window for washing or wiping the window surface and possibly drying the protective window with a blast of air. For example, EP-A-0688498 describes a cylindrical optical detector used in a milking arrangement that has a sliding cover adapted to move over a window. The cover is equipped with a sponge that is wetted by a cleaning liquid. As the cover slides over the window, the window is wiped by the sponge. U.S. Pat. No. 4,843,561 describes an agricultural harvester with an imaging devices in the form of cameras. These cameras are cleaned by means of a sprayer that sprays cleaning solution and wiper blades that automatically wipe a glass cover that protects the camera. A problem with such arrangements is that cleaning takes a certain time to accomplish and during this time the optical detector is necessarily inoperative, as the application of any form of liquid onto the surface will impede the correct operation of the detector.
U.S. Pat. No. 4,240,691 proposes a system for protecting television cameras that are used for surveillance. This essentially consists of a device that is mounted on the front of the camera objective and is shaped to generate air channels through which pressurised air is piped permanently. The protective device is designed to generate a laminate air flow across the surface of the camera objective so that any dust in the air is carried away from the surface of the objective before it can land on the glass. The cameras equipped with this device are operated continuously and can be unattended for many days, weeks or even months. The permanent air flow is intended to ensure that the cameras remain substantially free of dust. However, the need for a continuous pressurised air flow makes the arrangement expensive. Moreover, while this device it is effective for preventing airborne dust or small dirt particles from obscuring the camera objective in a location that is not heavily contaminated, for example fixed to a wall of a building well above ground level, such an arrangement would be less effective when located close to an inherently dirty floor, as the forced air flow would generate a turbulent flow that could throw up dust, particles, mud and liquid from the floor into the air. This would not only obscure the objective, but could also damage it, as any abrasive particles in the resultant dust cloud could scratch the camera over time. Such an arrangement is thus inherently unsuitable for use close to the floor of barns or milking parlours, for example, on a camera located roughly at the level of a cow's teats.
In view of the drawbacks of the above arrangements and methods, it is an object of the present invention to reduce the inoperative time due to cleaning of a camera that is arranged for use in a dirty environment, while maintaining reliable and effective operation by ensuring that the camera window remains clean.